In recent years, optical information reproduction apparatuses for optically reading information from recording media such as a compact disc (CD) and a DVD have been often used. In these recording media, information is recorded on information tracks having a fine width, and precise tracking control is generally required to reproduce the information from the information tracks. A tracking error needs to be detected to carry out the tracking control, and an optical means is generally used to carry out the detection. There are several known methods for optically detecting a tracking error. In one method, a photoelectric detector including divided light-receiving cells is used. Electric signals are output from light detected by each of the light-receiving cells, and a tracking error is detected from a phase difference between the output signals. Hereinafter, this method is referred to as a “phase difference method”. Further, there is also known a three-beam method in which, in addition to a photoelectric detector for receiving a main beam for reading information, a photoelectric detector for receiving a pair of sub beams for detecting a tracking error is used to detect a tracking error from a difference in level between the signals.
One of the major advantages of the optical information reproduction apparatus is that an access time to the target information is short. In order to make use of this advantage, it is important how quickly and stably a tracking servo is carried out on the target information track.
Hereinafter, an example of a tracking apparatus for the above-described conventional optical information reproduction apparatus will be described with reference to the drawings.
FIG. 4 shows a tracking control signal detector in the tracking apparatus for the conventional optical information reproduction apparatus employing the phase difference method. In FIG. 4, reference numeral 1 denotes a photoelectric detector, 2 denotes an arithmetic unit, 4 denotes a phase comparator, 5 denotes a low-pass filter, 6 denotes a comparator, 7 denotes a preamplifier, 8 denotes an absolute value detector, and 9 denotes a level detector.
Hereinafter, a configuration and an operation of the conventional tracking control signal detector shown in FIG. 4 will be described.
As shown in FIG. 4, the photoelectric detector 1 includes four light-receiving cells A, B, C and D that are separated from one another by a parting line substantially parallel to the direction in which the mapping of an information track extends and by a parting line substantially perpendicular to the parting line. The outputs from these four light-receiving cells are calculated by the arithmetic unit 2 so that a tracking phase difference can be detected. Consequently, the arithmetic unit 2 outputs at least a pair of arithmetic signals. The phase comparator 4 compares the phases of the pair of arithmetic signals and outputs a tracking phase difference signal (signal J). Further, the low-pass filter 5 removes a ripple component from the signal J and outputs a tracking error signal. The comparator 6 compares the tracking error signal with a reference level, and thus can output a cross signal indicating that a light spot has crossed an information track (JP S52-93222A, JP S57-181433 A).
The absolute value detector 8 receives the output of the phase comparator 4 and outputs a tracking phase difference absolute value signal (signal K). The level detector 9 detects a predetermined number of times or more the signal K has become larger than a predetermined value, and outputs a signal (so-called off-track signal) at the “L” level when the light spot is positioned on an information track or at the “H” level when the light spot is positioned between information tracks (JP 2701322 A). Terms other than the “off-track signal” may be used to describe the signal that indicates whether or not the light spot is positioned on an information track. In this specification, the signal is referred to as an “off-track signal”, as described above.
An information reproduction signal is obtained from the output of the photoelectric detector 1 and amplified by the preamplifier 7 into an RF signal.
Across signal is used to count the number of tracks skipped by the light spot for reading information when accessing the target information track or to detect track skipping in trick play such as a pause operation. The off-track signal is used, for example, to determine the stability of a tracking servo.
A direction signal that indicates a direction in which the light spot moves when accessing the target information track can be detected from the phase relationship between the cross signal and the off-track signal. By using this direction signal, a tracking breaking device capable of converging mechanical vibrations of the tracking apparatus in a short time can be operated. Moreover, if waveform shaping is carried out by using the cross signal and the off-track signal to remove edge noise, the number of skipped tracks can be counted more accurately.